1. Technical Field
The present invention relates to a sound collection apparatus, and more particularly to a sound collection apparatus for collecting, with enhanced accuracy, only a target sound generated by a target sound source.
2. Background Art
Conventionally, widespread is a technique of collecting only a sound received from a specific direction and preventing collection of a sound received from a direction other than the specific direction, by utilizing a directivity of a microphone. Further, suggested is a technique of extracting only a sound generated in a specific region, instead of a sound received from a specific direction, by using the technique as described above (see, for example, Patent Document 1).
Hereinafter, a conventional sound collection apparatus in which the technique of extracting only a sound generated in a specific region is realized, will be described with reference to FIG. 17. FIG. 17 is a diagram schematically illustrating signal processing performed by the conventional sound collection apparatus. As shown in FIG. 17, a sound collection section 91 and a sound collection section 92 are each configured as a microphone array having a directivity. A sound source S shown in FIG. 17 is a sound source, positioned at a predetermined position, for generating a target sound to be collected. The sound collection section 91 is positioned such that the sound source S is positioned on a primary axis a910 representing the directivity of the sound collection section 91. A secondary axis a911 and a secondary axis a912 are each an axis oriented such that sensitivities are each −6 dB when a sensitivity to a sound received from the direction indicated by the primary axis a910 is 0 dB. A range between the secondary axis a911 and the secondary axis a912 is a range in which the sound collection section 91 indicates a sensitivity of −6 dB or more, and is a range of a main beam of the sound collection section 91. The range of the main beam of the sound collection section 91, which corresponds to the width of the main beam, represents an angular width between the secondary axis a911 and the secondary axis a912, and varies depending on an acuteness represented by the directivity of the sound collection section 91. The sound collection section 92 is positioned at a position different from that of the sound collection section 91 such that the sound source S is positioned on a primary axis a920 representing the directivity of the sound collection section 92. A secondary axis a921 and a secondary axis a922 are each an axis oriented such that sensitivities are each −6 dB when a sensitivity to a sound received from the direction indicated by the primary axis a920 is 0 dB. A range between the secondary axis a921 and the secondary axis a922 is a range in which the sound collection section 92 indicates a sensitivity of −6 dB or more, and is a range of a main beam of the sound collection section 92. The width of the main beam of the sound collection section 92 represents an angular width between the secondary axis a921 and the secondary axis a922, and varies depending on an acuteness represented by the directivity of the sound collection section 92.
A region A9 indicated by the horizontal lines is an overlap region in which the main beam formed between the secondary axis a911 and the secondary axis a912 and the main beam formed between the secondary axis a921 and the secondary axis a922 overlap each other. The region A9 includes the sound source S.
The conventional sound collection apparatus shown in FIG. 17 initially divides, into a plurality of frequency bands, a frequency band of a collected-sound signal of a sound collected by the sound collection section 91. Further, a frequency band of a collected-sound signal of a sound collected by the sound collection section 92 is also divided into a plurality of frequency bands. Next, the conventional sound collection apparatus subjects the collected-sound signals of the frequency bands obtained through the division to logical operation, so as to extract only a signal of a sound generated in the region A9. The region A9 includes the sound source S, and therefore the extracted signal includes a sound generated from the sound source S. Thus, the conventional sound collection apparatus extracts only the sound generated in the region A9, so as to collect only a target sound generated from the sound source S.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-204092 (FIG. 2 and the like)
Here, a case where another sound source is provided, in the region A9 as described above, at a position other than that of the sound source S will be described. A sound generated from the another sound source is different from a target sound, and is a so-called disturbing sound. In this case, even when only a sound generated in the region A9 is extracted, the extracted signal may include the disturbing sound generated from the another sound source. Once the extracted signal includes a disturbing sound, it is technically difficult to separate the disturbing sound from the target sound. Therefore, as an alternative method for collecting, with enhanced accuracy, only the target sound generated from the sound source S, suggested is a method for reducing the size of the region A9 such that the another sound source is outside the region A9. In this method, it is necessary to reduce the width of a main beam of each of the sound collection section 91 and the sound collection section 92, and therefore the directivity of each of the sound collection section 91 and the sound collection section 92 needs to represent enhanced acuteness.
However, in order to enhance the acuteness represented by the directivity, it is necessary to increase the size of the microphone array forming each of the sound collection section 91 and the sound collection section 92. As a result, when, for example, the microphone array is allowed to have only a limited size, the enhancement of the acuteness represented by the directivity is limited.
Further, a case where each of the sound collection section 91 and the sound collection section 92 is configured as a microphone array of the superdirectivity of a secondary sound pressure gradient type so as to enhance the acuteness represented by the directivity will be described. In this case, the sound collection section 91 represents a polar pattern as shown in, for example, FIG. 18. FIG. 18 is a diagram illustrating a polar pattern represented by the sound collection section 91. The solid line in FIG. 18 represents the polar pattern, and represents a characteristic of a sensitivity varying in accordance with the direction from which the sound is received. Further, FIG. 18 shows the sensitivities for all directions (360 degrees). Furthermore, FIG. 18 shows a polar pattern obtained when the sound source S (not shown) outputs a target sound of a predetermined frequency (for example, 1 kHz). Further, in FIG. 18, the primary axis a910 represents 0 degree, and the sensitivity is 0 dB at the primary axis a910. The width of the main beam of the sound collection section 91 represents an angular width between the secondary axis a911 and the secondary axis a912, as described above. In FIG. 18, the width of the main beam is large and represents 90 degrees. Therefore, even when the microphone array of the superdirectivity is used, enhancement of the acuteness represented by the directivity is limited.
Thus, the enhancement of the acuteness represented by the directivity is limited, and therefore it is difficult to sufficiently reduce the size of the region A9 in which the main beam of the sound collection section 91 and the main beam of the sound collection section 92 overlap each other. As a result, the extracted signal may include a disturbing sound from another sound source, and it is difficult to collect, with enhanced accuracy, only the target sound from the sound source S.
Therefore, an object of the present invention is to provide a sound collection apparatus capable of collecting, with enhanced accuracy, only a target sound generated from a target sound source.